2:33pm UK, Tuesday August 31, 2004

Astronomers in Europe and the USA have discovered four new planets in just one week.

The flurry of activity capped the end of an intense period of planet hunting with significant results from three different teams.

180 new planet artist impression

Artist's impression of a new planet crossing orange star

None of the four planets are in Earth's solar system but at least two are similar in size to Neptune and Uranus.

The third and fourth are around the size of Jupiter, more than 300 times the size of Earth.

Scientists believe they are closing on to one of the ultimate discoveries - a planet similar in size and characteristics to Earth.

David Charbonneau, whose team at a Harvard-Smithsonian institute in Massachusetts earlier found a separate planet, said: "They've finally broken through to a new level."

The American discoveries sandwiched one made by a team at Switzerland's University of Geneva.

Michael Mayor and Didier Queloz said their planet is the smallest to have been found outside Earth's solar system.

None of the new planets can be seen with even the most powerful telescopes because they are hidden by the glare of nearby stars.